The present invention relates to magnetic resonance imaging (MRI), and more specifically, to Overhauser MRI (OMRI), which uses the Overhauser effect to enhance the nuclear magnetic resonance signal being received by the MRI apparatus and processed into an image. MRI is nowadays one of the most important modalities for medical diagnostic imaging, and it is described in a multitude of medical and technical textbooks. MRI is based on magnetic resonance of nuclei in magnetic fields. The resonance is manifested as an electromagnets signal, an "echo", received by the MRI apparatus as a response to an "exciting" signal, transmitted to the region to be imaged. The resonating nuclei, usually protons, have to be in a magnetic field (B.sub.0) to produce the resonance. The received echo signal contains information of the spatial position of the protons thanks to gradient fields superposed on B.sub.0 by gradient coils. An MRI apparatus thus typically contains means for producing the B.sub.0 : field, means for producing time varying gradient fields, means for transmitting and receiving electromagnetic signals at the nuclear magnetic resonance (NMR) frequency, and a master computer which operates said means sequentially to transmit and receive the NMR signals, and convert them to numbers which are mathematically interpreted and displayed as images. This automatic process is termed an "imaging sequence". MRI machines are presently commercially available from about ten different manufacturers.
The use of the Overhauser effect to enhance and modify MRI images is based on something which is also, more descriptively termed dynamic polarization, or double resonance. The quality of an MRI image is to a large extent dependant on the signal to noise ratio (s/n) of the received NMR signal. This ratio is proportional to the degree of polarization along B.sub.0 of the protons (or other nuclei being imaged). The polarization, normally proportional to B.sub.0, can be increased "dynamically" by a second magnetic resonance involving electrons, a "double resonance" in other words. Thus the use of the Overhauser effect implies performing a second magnetic resonance process, termed electron spin resonance (ESR), also called electron paramagnetic resonance (EPR). An OMRI instrument for producing MRI images enhanced by the Overhauser effect thus needs all the means included in a conventional MRI machine, and in addition means for transmitting the ESR signals to dynamically polarize the protons in the region of interest being imaged. To get an enhanced image one needs, furthermore, paramagnetic electrons in the region of interest. These can be provided by injecting a special contrast medium.
Machines for taking OMRI images are presently being developed and are not commercially available. A good article reviewing the present state of the art is "Proton-Electron Double Resonance Imaging of Exogeneous and Endogeneous Free Radicals in-vivo" D. J. Lurie and I. Nicholson published in: "Proceedings of International School of Physics Enrico Fermi, Course CXIII, Nuclear Magnetic Double Resonance"; Maraviglia B., ed; pub. Italian Physical Society; distr. North Holland, 1993.
A critical part of an OMRI instrument is the one closest to the patient being imaged, which emits the electromagnetic fields forming the transmitted ESR and NMR signals, and receives the NMR echo. This part, which we will call the "transducer", typically consists of a coil for the NMR signals, which oscillate in the radio frequency (RF) range, and some sort of antenna for the ESR signals, which operate in the very high frequency (VHF) range. Alternative names for the VHF antenna are resonator or applicator. At a typical field of 0.01 T the NMR frequency is about 400 kHz and the ESR frequency 300 MHz.
The present inventor has described the requirements on the VHF part in a previous application "VHF Applicator for Magnetic Resonance Imaging", GB-A-2246201. This application discloses an invention teaching how to make efficient VHF applicators based on electrical periodic structures, and discusses the requirements placed on such devices. It does not, however, address the difficulty encountered when integrating the RF and VHF parts of the complete OMRI transducer. This problem is solved by the present invention.
The requirements on the RF part of the transducer are that it should be sensitive and give even-density images from the region of interest. Technically this translates to:
Good filling factor. PA1 Low losses, meaning a high Q-value. PA1 It should produce a homogeneous B.sub.1 -field. PA1 It should be sensitive to a rotating B.sub.1 -field, not an oscillating one. PA1 The applicator should produce a rotating B.sub.2 -field. PA1 It should produce as little extra electric field as possible. PA1 It should have low losses, high Q-value. PA1 Fair filling factor. PA1 Homogeneous B.sub.2 -field.
The last requirement is for sensitivity: The magnetic resonance signals comprise rotating fields. An oscillating field is physically equivalent to two counterrotating fields of equal magnitude. If the system is made sensitive to an oscillating signal it will pick up signal from only one of the corresponding rotating components but noise from both of them, leading to a loss in s/n.
The requirements on the VHF applicator are that it should produce a high degree of dynamic polarization at a low level of heat dissipation in the region of interest. These two are contradictory, and the dissipated heat is a safety aspect when imaging humans and thus a very important one. Technically the requirements mean that:
The requirement for a rotating field, as opposed to an oscillating one, is usually more important for the VHF than for the RF signal. Eliminating the counterrotating field component means, in the former case, that the active component can be doubled (assuming that the limit is set by VHF dissipation in the patient). This increases the polarization, and thus the s/n by a factor 2. In the latter case the noise power is halved, but this increases s/n by just the square root of two. An optimal design of the applicator would place all the losses in the patient and at the same time shape the B.sub.2 field in such a way so as to produce maximal polarization without exceeding the allowed level of dissipation.
The listed requirements are difficult enough to fulfil in stand-alone RF coils and VHF applicators, but when combining the two, new problems occur. To keep a good filling factor for the RF coil it is essential to place it as close as possible to the patient, otherwise much of the received signal is lost. The RF coil should thus be inside the VHF applicator. Normally, however, the RF coil is not transparent to the VHF field but tends to act as a shield. In the cited paper by Lurie and Nicholson is presented the presently best attempt to address these problems. They describe a combination of a solenoidal coil for receiving the NMR signal, surrounded by an Alderman-Grant ESR resonator. In this transducer the B.sub.2 -field can penetrate the NMR coil because it is perpendicular to the axis of the coil, thus the magnetic flux lines can sneak in between the coil turns.
This transducer has the following fundamental shortcomings: First, both the B.sub.1 - and the B.sub.2 -fields are oscillating rather than rotating. Second: The electric field associated with the ESR magnetic field is tangential to the wire of the NMR coil in part of the coil. The importance of rotating fields has been explained before. To understand the significance of the latter drawback we have to consider the shape of the magnetic and electric fields in the transducer. The ESR resonator is designed to give a magnetic field which is predominantly homogeneous at the NMR coil. The associated electric field will, when the NMR coil is not present, be perpendicular to the magnetic one. The electric field lines will form approximate circles around the axis going through the resonator midpoint in the B.sub.2 -field direction. When the NMR coil is placed in the resonator we find that it will interact with the VHF electric field: Looking at that side of the coil where the B.sub.2 -field is approximately perpendicular to the wires we find that, as we trace the electric field lines, they will here run tangentially to the wires for about half of their extent. Thus the electric field will induce a voltage into the NMR coil wire, causing, in turn, a current. As long as the coil is rather small, this will be tolerable. The induced voltage is balanced from the opposite side of the coil, where a voltage with equal magnitude is induced, and the currents will be local and small enough to be insignificant.
If, however, the design is scaled up, for instance to accommodate the human head, problems will come. The coil wire will have an extension of several wavelengths of the VHF signal. It will therefore support many parasitic resonant modes close to this frequency, the number growing with the length of the wire. Many of these modes will correspond to a current distribution which does not have the same phase at diametrically opposite points of the coil, this feature also increases with size. The voltages induced from the VHF field will then no longer balance, and as they couple to resonances they will induce large currents. As a result the VHF power will be absorbed and the field distorted.